If the fan pressure rise is not known it can be calculated approximately from Specific Fan Power (SFP) data using:
Delta P = 1000 * SFP * Fan total efficiency
Annex E of ISO 5801 shows that by rearranging the formula it can be derived that the SFP is a function of fan pressure divided by the efficiency of the fan system. Therefore the SFP will increase or decrease with a respective increase or decrease in the system pressure.
The SFP is a function of the volume flow of the fan and the electrical power input and is quoted for a particular flow rate;
SFP = Pe/ V
Where:
V is volume flow (l/s)
Peis electrical power input (W) to the fan system or complete air movement installation
[Reference FMA, UK, 2006]
Typical values for various system types are shown in the table below.
|
System Type |
Specific Fan Power (W/l-s) |
|
Central mechanical ventilation including heating, cooling and heat recovery |
2.5 |
|
Central mechanical ventilation including heating and cooling |
2.0 |
|
All other systems |
1.8 |
|
Local ventilation units within the local area, such as window/wall/roof units, serving one room/area |
0.5 |
|
Local ventilation units remote from the local area, such as ceiling void or roof mounted units, serving one room/area |
1.2 |
|
Fan coil units (rating weighted average) |
0.8 |
Source ESTA.
Note: In reality the pressure rise across the fan has to meet the pressure drop across the index circuit of the actual system (i.e. the circuit with highest resistance to airflow). A more accurate estimate of fan pressure can be obtained by estimating the overall length of the ducting index circuit, assuming it’s sized on 1 Pa/m, adding approximately 20% for fittings and pressure drops for specific items such as diffusers, HEPA filters, etc.